This invention relates to the burning of fuel products, and more particularly is directed to rendering a combustion process in compliance with an applicable SO.sub.X :Btu standard. The invention is particularly concerned with the upgrading of subbituminous coal and/or lignite fuel products so that they comply with such a standard.
At the present time, the Federal Government and many states have imposed SO.sub.X :Btu standards for the burning of fuels. As noted in Report on Sulfur Oxide Control Technology, published in 1975 by the U.S. Department of Commerce, Commerce Technical Advisory Board, The Environmental Protection Agency (EPA) has adopted a national standard which limits SO.sub.2 emissions by each "new" coal-fired (i.e., installation commenced after August 1971) stationary source to 1.2 lbs. of SO.sub.2 per million Btu of heat input. The panel that published this report considered the following five potential near-term options for continuously meeting emissions standards of coal-fired electricity generating plants:
1. Use of low sulfur coal (Eastern & Western) PA1 2. Use of coal beneficiation (cleaning) PA1 3. Use of sludge-producing flue gas desulfurization (F.G.D.) PA1 4. use of coal beneficiation combined with flue gas desulfurization (F.G.D.) PA1 5. use of regenerative F.G.D.
None of these alternatives gave serious consideration to the use of subbituminous coal and/or lignite fuel relatively high in total moisture content. In fact, as pointed out in the publication Burning the Sub-bituminous Coals of Montana and Wyoming in Large Utility Boilers, by R. J. Gray and G. F. Moore, published by The American Society of Mechnical Engineers (1975), the lower heat content of lignites and subbituminous coals "creates a problem, primarily because it takes more coal for a given heat input." As also noted, because of their low calorific values, some of these coals will not meet the Federal air pollution requirement of 1.2 lbs. of SO.sub.2 per million Btu heat input, even though they are low in sulfur content.
Electric utility companies have accordingly found that they are unable to use some western subbituminous coal and lignite fuel products as they are presently produced.
On the other hand, some electric utility companies have decided in order to use some western subbituminous coal and lignite fuel products that they will install stack gas scrubbing systems in order to comply with EPA regulations related to sulfur dioxide.
The present invention is directed to the use of such fuel products, and specifically to upgrading them from a condition in which they are non-complying with the applicable SO.sub.X :Btu standard to a condition in which the product is in compliance during the combustion process. This invention provides such upgrading by treatment at the combustion site. The invention is particularly concerned with such upgrading of a fuel product without an associated high cost.
The present invention proceeds on the basis that an applicable SO.sub.X :Btu standard may be met raising sufficiently the heating value of the fuel product without necessarily resorting to sulfur removal processes so that the SO.sub.X :Btu standard will be met during the combustion process. Thus this invention primarily approaches the problem from the standpoint of raising the heating value rather than lowering the sulfur content.
Recognizing that total moisture is the major cause of relatively low heating values of western subbituminous coals and lignite fuel products, the present invention contemplates drying a fuel product at the combustion site to lower significantly the total moisture content of the fuel product and to concomitantly raise it sufficiently so that the SO.sub.X :Btu standard will be met during the combustion process. Additionally, the product may be "cleaned" i.e., made to undergo particle weight separation so that heavier weight ash particles containing sulfur are removed. In this fashion some sulfur reduction is achieved as well as increasing the heating value of the fuel product, all to contribute toward the lowering of the SO.sub.X :Btu ratio so that it is in compliance with the applicable emission standard during the combustion process.
Fuel drying has taken place at the combustion site in the past. Many utilities recognize the fact that increased pulverizer efficiencies may be obtained if the fuel product is dried prior to introduction to the pulverizer. To this extent, fuel drying is presently being practiced by many utilities. However, there is no appreciation of the fact that a non-compliant fuel product (non-complying with applicable SO.sub.X :Btu standards) may be upgraded so that it is a compliant fuel product by the use of fuel drying. Kreisinger et al U.S. Pat. No. 1,608,699 is typical of a prior art system utilizing drying ahead of a pulverizer to increase pulverizer efficiency. A system such as shown in that patent would be suitable for the processing of subbituminous coal and lignite fuel products that need only be dried in order to meet the applicable SO.sub.X :Btu standard. Kreisinger, however, does not provide for cleaning.
In the practice of the present invention, waste heat from the combustion process itself may be utilized in a recycling thereof for the purpose of carrying out the drying operation. Additionally, since the drying of the fuel product increases the efficiency of the pulverizing unit, advantage may be taken of this increased efficiency by utilizing pulverization at different stages during the treatment process, for example, providing an initial pulverizing operation, followed by a separation of heavier weight sulfur-containing ash particles, followed by a final pulverizing operation.
The invention will be more completely understood by reference to the following detailed description, when taken in conjunction with the appended drawing.